1. Field of the Invention
The present invention relates to a multistroke punching method and an apparatus for punching a workpiece at multistrokes, and more specifically to a method of punching a workpiece at multistrokes without producing burr, rollover, sags, scar, etc., and a hydraulic punch press for punching a workpiece in accordance with the multistroke punching method.
2. Description of the Prior Art
When a slot or a circular hole is formed in a sheet metal workpiece with a press machine such as a turret punch press, conventionally a die having a rectangular or circular edge is moved down toward the workpiece to punch the workpiece at a single stroke or a single strike.
This is because it is rather difficult to adjust or change the stroke of a ram for striking the punch in the case of the conventional turret punch press. Therefore, there exists a problem in that a slot or a hole formed in a sheet metal workpiece is subjected to various defects such as burr, rollover, sags, scar, etc. due to deformation of the cut-off plane of the workpiece, setting errors of the punch and die, or other reasons such as temperature difference or vibrations.
As a result, the punched workpiece should be finished to remove the above-mentioned defects, thus increasing the number of processing steps and therefore cost, and decreasing the precision of the punched product. In addition, there arise another problem such that the product is damaged when the punched product is being finished.